1) Field of Invention
The present invention generally relates to methods of detecting frequency hopping communications signals in the presence of clutter.                2) Description of the Prior Art        
Acoustic communication between underwater platforms is often desirable. FIG. 1 and the ensuing text provide a brief explanation of this type of communication. A signal source 10 is joined to a transducer 12 to produce an encoded acoustic signal through water 14. The acoustic signal 16A, 16B and 16C travels through the water 14 to a sensor 18, such as a hydrophone, joined to a receiver 20.
As shown, the acoustic signal can follow several paths. A direct path 16A through an acoustic channel 22 is preferred. Acoustic channel 22 is the region of water 14 defined by thermal boundaries 24A and 24B. The acoustic signal can also travel in path 16B where it bounces off water 14 surface 26 or in path 16C where it bounces off water 14 bottom 28. Other paths are also possible. Each of these paths will arrive at sensor 18 at a different time. Some paths may interfere with later transmitted acoustic signals.
The acoustic signal can be spread across multiple acoustic frequencies in a technique known as frequency hopping. In this technique, a signal is transmitted at several different frequencies. For example, four fixed length signals positioned sequentially in time at frequencies 1000 Hz, 1040 Hz, 1020 Hz and 1010 Hz are detected and values are assigned to represent the numbers 0421 by sequence of transmission and frequency variation. The numbers correspond to a message in a codebook shared between the transmitter and the receiver.
A standard method for decoding messages transmitted by frequency hopping is by correlating the entire data sequence with replicas of all possible time-frequency combinations that represent possible messages. The method then picks the time-frequency combination that best corresponds with a message. Best correspondence can be determined by correlation magnitude, signal strength or some other metric. Distortion from source/receiver motion and complex acoustic propagation can be handled by estimation of channel properties and pre-filtering with inverse transfer functions. Under benign circumstances such as with white noise, this approach may allow message decoding at a low signal-to-noise ratio. However, performance can be limited in the presence of interference from other sources that may overlap with communications waveforms.
As can be seen, there is a need for an acoustic receiver for underwater digital communications that provides robust performance in the presence of multipath signal propagation and clutter.